1. The problem asks to approximate the number of children living with only their grandparents in 1991, which is $f(1991)$. 2. From the graph description, the y-axis shows the number of children in millions, and the x-axis shows years from 1991 to 2009. 3. The graph starts slightly above 1.3 million in 1991. 4. Therefore, the approximate value of $f(1991)$ is about 1.35 million children. 5. This means around 1.35 million children lived with only their grandparents in 1991 according to the graph.